When the stock market has upward momentum, it tends to keep going up. However, downward momentum, like we've had since August, tends to be followed by sideways movement with high volatility, as this post from the MarketSci blog shows.

It's better to be in Treasury bills than in the market when there's negative momentum. This is both for the lower volatility and to earn interest.

For small investors, this could mean moving to a money market fund until the stock market starts going up again.